This invention relates to the catalytic reduction of nitrogen monoxide (NO) from highly oxidizing environments. Such catalytic reductions of this type employ the use of small quantities of methane (CH.sub.4) to provide a selective catalytic reduction (SCR) of the NO.
2. Description of the Related Art
Although the catalytic reduction of nitrogen oxide (NO) by hydrocarbons, carbon monoxide (CO), carbon (C), and hydrogen (H.sub.2) have been reported under oxidative conditions, the selective catalytic reduction (SCR) process is the only commercially proven process where NO can be selectively reduced (instead of O.sub.2) under the highly oxidative conditions typically from stationary combustion sources whose exhaust temperature is usually less than 1200.degree. F. Although SCR is a proven commercial process for the reduction of nitrogen oxides (NO and NO.sub.2) from highly oxidizing environments, it requires the use of ammonia (NH.sub.3) as the reducing species.
Using ammonia as a reducing agent has several disadvantages:
(1) NH.sub.3 is toxic, corrosive and difficult to handle; PA1 (2) NH.sub.3 must be transported to the power plant; PA1 (3) Unreacted NH.sub.3 that may bypass the catalyst; (ammonia slippage) is a major concern. PA1 (4) Most NH.sub.3 based SCRs have an upper operating temperature of approximately 450.degree. C. Higher operating temperatures cause the NH.sub.3 to be oxidized to form NO; and PA1 (5) In sulfur laden streams, NH.sub.3 reacts with sulfur oxides to form ammonium sulfates which may corrode the exhaust pipes.
Until recently, no data was available to indicate that hydrocarbons could react selectively with NO under highly oxidizing conditions. It was found that instead of reacting with NO, the hydrocarbons would instead be consumed via oxidation reactions with the more abundant oxygen molecules. In the past few years, however, there has been a large number of published reports on the use of hydrocarbons for NO reduction in oxidizing environments.
In 1990, Hamada et al. published a letter entitled "Highly Selective Reduction of Nitrogen Oxides with Hydrocarbons over H-form Zeolite Catalysts in Oxygen-Rich Atmospheres", Applied Catalysis, Volume 64, 1990, pages L1-L4,indicating that some zeolites (especially H-ZSM-5 and H-mordenite) were observed to be active for the catalytic reduction of NO in 10% oxygen (optimal temperature approximately 400.degree. C.) by using propane or propylene as a reducing species. In a following letter by Kintaichi et al., entitled "Selective Reduction of Nitrogen Oxides with Hydrocarbons over Solid Acid Catalysts in Oxygen-Rich Atmospheres" Catalysis Letters, Volume 6, 1990, pages 237-244, it was shown that NO can be reduced by propane over metal oxides (especially Al.sub.2 O.sub.3). In 1991, Misono et al in an article entitled "Catalytic Removal of Nitrogen Monoxide over Rare Earth Ion-Exchanged Zeolites in the Presence of Propene and Oxygen", Chemistry Letters, 1991, pages 1001-1002, it was reported that Ce- and Pr-exchanged ZSM-5 and Y-type zeolites to be active for the catalytic reduction of NO with propylene in 2% oxygen (optimal temperature approximately 400.degree. C.). Kikuchi et al. in an article entitled "Selective Reduction of NO with Propylene on Fe-silicate Catalysts", Chemistry Letters, 1991, pages 1063-1066 reported the catalytic reduction of NO with propylene over Fe-silicate catalysts. The focus of the Kikuchi et al study was to demonstrate that Fe-silicates were less susceptible to SO.sub.2 poisoning than Cu-ZSM-5 at low temperatures (less than 400.degree. C.). Hamada et al. in another letter entitled "Sulfate-promoted Metal Oxide Catalysts for the Selective Reduction of Nitrogen Monoxide by Propane in Oxygen-rich Atmosphere", Chemistry Letters, 1991, pages 2179-2182, reported that an active NO reduction catalyst (with propane) could be produced by pretreating TiO.sub.2, ZrO.sub.2, and Fe.sub.2 O.sub.3 with sulfuric acid. Sato et al. in al article entitled "Cu-ZSM-5 Zeolite as Highly Active Catalysts for Removal of Nitrogen Monoxide from Emission of Diesel Engines", Applied Catalysis, Volume 70, 1991, pages L1-L5 reported that ethylene could be used for NO reduction over various cation exchanged zeolites. Ethylene was used because it was said to be the predominant hydrocarbon found in diesel emissions. In 2% oxygen, Sato's group measured NO conversion as a function of temperature for various cation (Cu, Co, H, Ag and Zn) exchanged ZSM-5 zeolites. It was found that the Cu-exchanged zeolite to be the most active at lower temperatures (approximately 250.degree. C.).
Although the recent publications have shown that some hydrocarbons, such as propane, propene, or ethylene can be used as reducing species for the catalytic reduction of NO (to N.sub.2), a more advantageous NO.sub.x reduction would be provided if methane (CH.sub.4) or natural gas could be used as a reducing agent for NO.sub.x reduction under highly oxidizing environments found from stationary combustion sources.
It is apparent from the above that there exists a need in the art for a catalyst which does not use ammonia, and which at least equals the reduction characteristics of known catalysts. An attractive alternative is the use of methane or natural gas as the reducing agent instead of NH.sub.3. It is a purpose of this invention to fulfill this and other needs in the art in a manner more apparent to the skilled artisan once given the following disclosure.